Silicon photonic devices are photonic devices that use silicon (Si) and its derivatives as an optical medium. Silicon photonic devices may be fabricated using existing semiconductor fabrication techniques for electrical devices. Thus, many silicon photonic devices are hybrid devices that have both optical components and electrical components on a single chip. Such a hybrid device may be part of a larger system in package (SIP), which refers to a number of integrated circuits in a single package.
SIPs with silicon photonic components often comprise both a silicon optical waveguide and an optical fiber. The waveguide often comprises both an input port and an output port. The optical fiber is often a single-mode fiber. There is a need to convert an optical mode of the waveguide to an optical mode of the fiber. In this context, an optical mode may refer to a shape of a light beam at the exit or entrance of a waveguide, fiber, or other medium. An optical mode size refers to the physical size of an optical mode. The physical size may be based on optical intensity. For instance, light below a specified threshold intensity may not be considered part of the physical size.
The mode size of the waveguide input port and output port is relatively smaller and typically about 2-4 micrometers (μm). The mode size of the fiber is relatively larger and typically about 8-10 μm. That relative size difference makes it difficult to couple light between the waveguide and the fiber.